1. Field of the Invention
The present general inventive concept relates to a fixing device usable with an image forming apparatus, and more particularly, to a fixing device usable with an image forming apparatus in which a manufacturing cost is reduced and credibility and stability on an overheating prevention operation are improved.
2. Description of the Related Art
An image forming apparatus, such as a laser printer and the like, has a charging process which rotates an electric charging roller adjacent to an opto photo-organic conductor (OPC) drum and charges a surface of the OPC drum, an exposure process which scans a laser beam from a laser scanning unit (LSU) to the surface of the OPC drum and forms a desired electrostatic latent image on the surface of the OPC drum, a developing process which supplies a toner to the surface of the OPC drum and develops the electrostatic latent image formed on the surface of the OPC drum into a toner image of a powder state which is an visible image, a transferring process which applies a predetermined transfer voltage to a transferring roller contacting the OPC drum with a predetermined pressure and the OPC drum formed with the toner image and transfers the toner image formed on the OPC drum to a recording medium passing between the transferring roller and the OPC drum, and a fixing process which heats the recording medium with the toner image and melts the toner image of the powder state to fuse the toner image on the recording medium employing a fixing device having a fixing roller. Therefore, the image forming apparatus forms the desired image on the recording medium according to the above processes.
In the fixing process, a heater is mounted inside the fixing roller and/or a fixing back up roller, as a heat source for fixing. Radiant heat from the heater heats a surface of the fixing roller in a predetermined temperature range.
FIG. 1 illustrates schematically a conventional fixing device 200 for an image forming apparatus. As shown in FIG. 1, the fixing device 200 includes a fixing roller 204 having a cylinder shape and formed with a layer coated with Teflon, etc., and a heater 206 mounted at a center of an inside of the fixing roller 204. The heater 206 generates heat inside the fixing roller 204, and radiant heat generated from the heater 206 heats the fixing roller 203.
In a lower part of the fixing roller 204 is mounted a fixing back up roller (not shown). The fixing back up roller is elastically supported by a predetermined elastic member and pressurizes a recoding medium passing between the fixing roller 204 and the fixing back up roller with a predetermined pressure for fixing the toner image on the recording medium.
The toner image of a powder state formed on the recording medium passes between the fixing roller 204 and the fixing back up roller to be pressurized and heated by the predetermined pressure and heat. Accordingly, the toner image is fused to the recording medium.
Near the fixing roller 204 is mounted a thermistor 208 which detects a surface temperature of the fixing roller 204 and outputs an electric signal as an output signal according to the detected surface temperature. The fixing device 200 comprises a power control switch 212 supplying and interrupting an electric power from a power supplier 202 to the heater 206 corresponding to a signal from a switch controller 210 according to the output signal of the thermistor 208.
The switch controller 210 turns on/off the power control switch 212 according to a comparison between a temperature detected by the thermistor 208 and a predetermined set temperature, thereby controlling a power supply to the heater 206. Therefore, the surface temperature of the fixing roller 204 is controlled enough to fix the toner image to the recording medium.
The fixing device 200 comprises a thermoswitch 214, such as a thermostat, that interrupts the electric power supplied to the heater 206, in case where the surface temperature of the fixing roller 204 reaches a predetermined overheating temperature. The thermoswitch 214 operates as overheating prevention means for protecting elements adjacent to the fixing roller 204 when the fixing roller 204 is overheated in a case where the thermistor 208, the switch controller 210, and the power control switch 212 can not control the surface temperature of the fixing roller 204.
Because the thermoswitch 214 of the conventional fixing device 200 is connected in series between the power supplier 202 and heater 206, the conventional thermoswitch 214 should have an electric power capacity corresponding to the electric power supplied to the heater 206. Particularly, in a case of the heater 206 calling for the high frequency power such an induction heating method, the supplied electric power increases and the electric power capacity of the thermoswitch 214 corresponding thereto also increases, thereby increasing manufacturing costs.
Therefore, there is needed a device to reduce the manufacturing costs of the fixing device and ensure credibility and stability on an overheating prevention operation.